Applying invisible covert marks to product packaging is a well establish method for authenticating products and thus combating counterfeiting. In addition, when variable invisible information is printed, batch-level and item-level tracking of products can be accomplished in a covert manner. Common covert marking materials are ultraviolet (UV) fluorescent inks. The security mark is invisible under normal lighting, but is revealed when a UV light source is used.
One limitation of this approach is that printed cartons and packaging materials can have a UV curable overprint varnish applied to help improve the durability of the surface, for example add scuff resistance; change the gloss finish of the surface; or protect the inks from unintentionally washing off or intentionally being removed, in the case of covert tracking information. These overprint varnishes (also referred to as overcoat varnishes) typically contain optical brighteners that can interfere with the UV fluorescence of the security marks. One example of such a varnish is InX International Procure™ UV 10090 LP overprint varnish.
U.S. Publication No. 2009/0104373 (Vanbesien) describes applying a radiation curable varnish to a document and authenticating the document via the radiation curable fluorescent varnish. However, in their case the hidden information is applied to the document by image-wise printing of the radiation curable varnish using a digital press. This can pose problems when there is an imperfect match of the gloss of the varnish with the gloss of the substrate which would make the invisible mark visible to the unaided eye as a gloss differential. This is undesirable because it exposes the hidden mark. Digitally printing a radiation curable ink can also pose hardware problems such as jetting of a high viscosity liquid and clogging of inkjet nozzles because of crosslinking of the varnish.
Another way of adding a security mark is to use a UV absorbing ink on a print surface that contains an optical brightener, thereby creating a negative (dark) image. This can work well on label stock, which often has optical brighteners. Typical carton stock used in packaging, however, often does not contain such optical brighteners; therefore this simple approach is not feasible.
It is therefore highly desirable to have a solution that allows covert embedding of information via a robust printing method on substrates that are not optically brightened, and that can also subsequently be treated with an overprint varnish to protect the package.